Generic Nature of Interfacial Phenomena in Solutions of Nonionic Hydrotropes

• Published in: Langmuir Vol. 35; no. 41; pp. 13480 - 13487
• Main Authors: Novikov, Andrei A, Semenov, Anton P, Kuchierskaya, Alexandra A, Kopitsyn, Dmitry S, Vinokurov, Vladimir A, Anisimov, Mikhail A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 15.10.2019
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/acs.langmuir.9b02004

Nonionic hydrotropes (low-molecular-weight amphiphiles) demonstrate striking dual actions in bulk solutions and interfaces, exhibiting both surfactant-like and co-solvent properties. We report on peculiar, strongly affected by this duality, liquid–liquid and air–liquid–liquid interfacial behavior in aqueous ternary systems, containing hydrotropes and hydrocarbons, in a broad range of compositions and at various temperatures. Phase diagrams of the studied systems, containing tertiary butanol (TBA), as a hydrotrope, are of Type 1: the hydrotrope, at the experimental conditions, is completely miscible with water and with all investigated hydrocarbons [cyclohexane (CHX), toluene (TOL), and n-decane (DEC)], whereas the ternary mixtures exhibit liquid–liquid phase separation terminated at corresponding critical points. The shape and location of the phase separation boundary are only weakly dependent on temperature and the hydrocarbon’s nature; however, the critical point in the water–TBA–DEC system is significantly shifted toward a higher TBA concentration. For the experimentally studied systems and for available data reported in the literature, we confirmed an apparently generic (for nonionic hydrotropes) phenomenon of a dual action at water–oil interfaces (earlier found in water–TBA–CHX [J. Phys. Chem. C 2017, 121, 16423]): at low concentrations, hydrotropes saturate the water–oil interface like a surfactant, whereas at higher concentrations they act as co-solvents, resulting in vanishing interfacial tension at the liquid–liquid critical point. We suggest a universal crossover function that accurately interpolates the two theoretically based limits of interfacial behavior. This crossover function also accounts for earlier deviations from Langmuir–von Szyszkowski limiting behavior in the water–TBA–DEC system, caused by lower solubility (relative to other studied hydrocarbons) of DEC in water. An intriguing correlation between the dual action of hydrotropes at the water–oil interface and the behavior of the liquid–air interfaces is also discussed.